How environmental signals are perceived by living organisms and integrated to modulate their key developmental regulators to control gene expression and development program is a fundamental question of biology. The long-term goal of this research is to dissect the molecular and cellular basis of how light signals are integrated to achieve the control of the development mode switch, employing the light-regulated seedling development of Arabidopsis thaliana as an experimental system. A multidisciplinary approach in our past effort have established working model that in darkness, COP1 (constitutively photomorphogenic 1) acts inside the nucleus to suppress the photomorphogenic development, via directly interacting with and negatively regulating specific transcription factors whose activities are responsible for promoting photomorphogenesis. While light abolishes COP1 nuclear accumulation and abrogates its repressive action. Here I propose to extend our current investigation through a combination of molecular genetic, cell biological, and biochemical approaches. To confirm our working model and reveal the molecular insights of COP1 action, we will further examine the regulatory role of the best defined COP1 and HY5 interaction in vivo, and to illuminate how this interaction translates into the regulation of HY5 stability, and its ability to interact with its target promoters and regulate their expression. To illustrate the molecular basis and mechanism of COP1's ability to interact with multiple targets and regulate pleiotropic development processes, we shall further analyze the role of four new putative downstream targets which specifically interact with the WD-40 repeats domain of COP1 and reveal how they specifically contact the distinct interactive surfaces defined by those seven WD-40-repeats. As a first step to understand how light regulate COP1 activity, we will further characterize a recently cloned signaling component which play a key role in phyA inactivation of COP1 and whose expression is also regulated by hormone auxin. To achieve a more comprehensive picture of this regulatory switch, we will pursue molecular cloning and characterization of two additional key regulatory components that defined by two separate genetic screens. If time permits, in vitro reconstitution system will be developed using our defined components and further genetic screens will be carried to reveal additional regulatory components. Those studies will provide novel insights into the mechanism of how an environmental signal achieves its control of an organism's development program. Since all the COP proteins defined so far are highly conserved among all multicellular eukaryotic organisms including human, the fundamental mechanism gained in this Arabidopsis model system will provide a guidance for understanding the function of those conserved regulators in human and thus enhance our ability to combat human health problems.